The present invention relates to a developing device for use in an image forming apparatus such as a copying apparatus or the like.
A developing device has been proposed in, for example, U.S. Pat. No. 4,615,606 which includes a developing sleeve 4 having a magnetic roller 5 incorporated therein, a toner supply roller 7 formed, on its surface, with minute recesses and confronting the developing sleeve 4 across a predetermined gap extending therebetween and a toner reservoir 3 disposed above or at a side of the toner supply roller 7 as shown in FIG. 1. The magnetic roller 5 comprising a plurality of magnets arranged in a circumferential direction of the developing sleeve 4 such that magnetic poles of the magnets extend in an axial direction of the developing sleeve 4.
In the known developing device referred to above, developer held on the surface of the developing sleeve 4 forms a magnetic brush along lines of magnetic force of the magnets provided in the magnetic roller 5 and the magnetic brush rubs against a surface of a member bearing an electrostatic latent image such that a visible toner image is formed on the surface of the member bearing the electrostatic latent image. A potential difference is established between the developing sleeve 4 and the toner supply roller 7 such that the developing sleeve 4 is replenished with an amount of toner corresponding to an amount of the toner to be consumed.
However, in the known developing device, the filling of the minute recesses of the toner supply roller 7 with new toner is facilitated by a scooping action of the minute recesses of the toner supply roller 7 and the device does not have means for positively filling the minute recesses with new toner. Namely, the known developing device has a drawback in that the filling efficiency is low and thus, the toner cannot be sufficiently supplied to the developing sleeve 4.
Thus, in order to improve the efficiency of filling the minute recesses with toner, the present inventors provided in the toner reservoir a rotary means for forcibly causing the toner to flow towards the toner supply roller. As a result, the filling efficiency of the toner was improved. However, electrically charged toner adhering to the toner supply roller falls into the toner reservoir by action of the rotary means and is mixed with new toner in the toner reservoir, consequently electrostatically aggregating the new toner. Thus, another problem arises in that when the aggregated toner is supplied to the developing sleeve so as to be used for development, the aggregated toner causes fog in a copied image.